This study aims to determine if amenorrheic trained females are more energetically efficient than eumenorrheic trained females. To determine this, 24 hr energy expenditure and its components (resting metabolic rate, thermic effect of food and energy cost of activity) will be measured using a wholeroom indirect calorimeter. Measurements will be made following a 6 day period of diet control. All food will be provided to subjects during this time and energy requirements for weight stability will be determined. Energy expenditure, fuel oxidation and post-exercise metabolic rate will be determined in subjects during and following a typical training session. This will be measured using a treadmill and indirect calorimetry.